The present invention relates to a method and apparatus for optical pickup focusing control, and more particularly, to a method and apparatus for optical pickup focusing control by a controller and a memory provided in a driving loop which can adapt to changes in circumstances and characteristics of the focusing system.
As shown in FIG. 1, in a conventional optical pickup, a laser diode 10, a collimating lens 30, a beam splitter 40 for dividing light projected from laser diode 10, an object lens 50 for focusing the beam reflected by beam splitter 40, and a disk 70 as an information recording medium are sequentially disposed. Also, the optical pickup has actuator 60 for driving the object lens. The light reflected from the surface of disk 70 is transmitted through beam splitter 40 or reflected from the beam splitter. The transmitted light is detected by photo-detector 80 containing an optical dividing board which is divided into at least two portions. If the optical dividing board has four dividing portions, the light transmitted to each divided portion is converted into a current signal and two signals generated by divided portions facing each other on the diagonal lines of the board are summed to be input into the input ports of differential operator 90 for comparing the difference of the input signals. Differential operator 90 detects the difference of the two input signals and converts the detected current signal into a voltage signal.
FIG. 2 is a waveform diagram representing an error according to the distance of the converted focusing voltage signal to be detected at point "a" of FIG. 1. X-axis and Y-axis coordinates represent a distance error and a voltage, respectively. The focusing error signal is amplified through amplifier 100 shown in FIG. 1 and drives object lens 50 in a direction minimizing the distance error using the amplified focusing error signal. Object lens 50 is driven by actuator 60. Control compensator 110 is provided adjacent to amplifier 100 for obtaining the control input necessary for controlling actuator 60. Driver 130 is provided between control compensator 110 and actuator 60 to drive actuator 60 according to the signal from control compensator 110.
As described above, the conventional focusing control apparatus needs a stable closed loop which is reconnected to actuator 60 by passing through actuator 60, photo-detector 80, differential operator 90, amplifier 100, control compensator 110 and driver 130 and should be designed so that the distance error occurring due to a fluctuation of disk 70 is within an allowable error range. As a result, it is difficult to design control compensator 110 since the allowable error range (.+-.0.5 .mu.m) is less than the movement of disk 70 (.+-.0.5 mm), requiring control compensator 100 to include many compensators. Also, there are problems in that the sensitivity of photo-detector 80 and actuator 60 used for obtaining the distance error signal may vary significantly so that the gain and offset of amplifier 100 has to be compensated by adjusting a variable resistance during assembly and in that the optical pickup system can be unstable if the sensitivity is varied due to changes in operating conditions such as temperature, the intensity of radiation of a light source and the like.